Az mi: usually asks when doesn't know the meaning of something the other said
Mi az: asks when sb knocks the door when on the toilet or similar place
Mi ez: ask when you see sth interesting/exciting if you don't know what it actually is
Ez mi: ask when you see sth nasty/obscene you want to know about
You should use "Ez mi?" and "Az mi?" only when you are able to point at the object in question. What is this? What is that?
You can also ask "Ez mi?" if you know that the listener is thinking about the same thing as you are, for example, right after you both sense something because you are able to point to something on your mind (1).
You can not say "Az mi?" if you cannot point at the thing you are talking about (2). However, "Az mi?" is correct when you are referring to a noun because you are able to point to something on his/her mind (3).
- "Te is hallod, amit én?" = Do you hear what I hear?
- "Ez mi?" or "Mi ez?" = What is it?
- "Szeretnék kérdezni tőled valamit." = I would like to ask you something.
- "Mi az?" = What is it?
- "Szereted a tiramisut?" = Do you like tiramisu?
- "Az mi?" or "Mi az?" = What is it?
As you can see, one-to-one translation is not always possible. Also, Hungarians say this, "Ez," in situations where an English-speaking person would use that:
- "Ez igazán finom volt. Köszönöm!" = That was really tasty. Thank you!
Ah, but what is the subject?
Note that in English it is "is this us?" and not "are this we?", indicating that the subject may be "this" and not "we".
In German, however, it would be "Sind wir das?" or "Sind das wir?" i.e. "Are we that? Are that we?"
I presume from your answer that Hungarian does it this way as well -- but it's not immediately obvious that it must be so.
Ahh, copula sentences. What a fun for everyone. :D
Arguably in English you can also say "Are we this?" and get the same meaning. English is the odd one out in those regards with putting the non-subject here in the accusative case. (I hesitate to call it 'object'.) In a copula sentence usually both clauses are in nominative.
Translating "Mi ezek?" (Or "Mi ez?", that would also be possible, it seems) as "Is this us?" would sound at least as weird as "Ist das wir?" in German. Point is, I don't know if it'd be technically correct, but if you try to translate "Is this us?" as "Mi ez?", you'll be always misunderstood.